Is it a "must have?" No. Is it an ingredient worth adding? Yes. Sulfated castor oil (also known as sulfonated castor oil or Turkey Red oil) is a water soluble form of castor oil that soapmakers often add to make liquid soaps especially mild and gentle to the skin.

The soap I make that uses sulfated castor to greatest effect is my liquid dog shampoo. Dogs, much more than people, need oil for their skin and coat to be healthy, and the sulfated castor seems to be especially helpful in preserving the right balance. The customer feedback has been 100 percent positive. Glowing even!

Castor bean seeds

﻿

When I decided to develop a bar version of the dog shampoo, however, I ran into a roadblock. Sulfated castor can be added to liquid soaps without being figured into the lye calculation because it's water soluble. Its saponification (SAP) value -- or the question of whether it has one -- doesn't seem to matter. But it does matter with a bar soap because it will potentially ruin the batch if it's not included in the lye calculation.

The simple question Does sulfated castor oil have a SAP value? was difficult to answer. I searched for the information every way I could think of. I checked a number of very reputable websites. Some assigned it the same SAP value as regular castor oil. Others gave it a SAP value of 0. But I couldn't find a third party source to susbstantiate either one. I left it out of the recipe.

Several weeks ago, I remade the dog shampoo bars, and before I did I searched once again and came up empty-handed once again. So I took a chance and added it to the recipe without figuring it into the lye calculation. I used a slightly lower lye discount just to be safe. The soap came to trace nicely, went into a good gel, and the next morning I had what appeared to be perfectly good -- albeit a little soft -- dog shampoo bars. (They are hardening nicely as they cure, I'm pleased to add.)

But I still didn't have an answer to my question and it was bugging the heck out of me. Then it hit me. There's one person I know who might actually have the answer.

I know someone who's writing the most in-depth book on the science of soapmaking that's ever been attempted. I'm on the list of soapmakers and others who've had the oppportunity to review drafts of the book, offer comments, look for typos, and generally be amazed (and, in my case, intimidated!) by the level of scientific scrutiny he brings to the process of soapmaking. When it's published, I guarantee that no serious soapmaker will be without it. An invaluable resource.

I met and talked to the author very briefly at a soapmakers convention a couple years ago, and on the basis of that very tenuous connection I e-mailed him my question. I hoped it was something he knew off the top of his head and that it would only take 5 seconds of his time to reply. What I received was considerably more detailed.

It turns out that he didn't have a definitive answer because he had hadn't yet had the opportunity to actually measure the SAP value of sulfated castor oil himself. And he also hadn't come across anything in his reading or studies that substantiated it one way or the other. But he had done a quick experiment, he said, that "led me to believe that sulfated castor oil is already saponified and therefore has a SAP close to 0." In the following paragraphs he lead me through the entire process, starting with the premise for the experiment, the proven facts that formed the basis for it, the hypothesis he was working with, and the experiement itself.

Long story short, he found that the sulfated castor reacted the way saponfied oil would react in the test he performed, not the way unsaponified oil would react. As he said, this is not a definitive answer, but it does suggest one.

Shortly after that I found some tantalizing information to support his idea. I came across several websites stating that once castor oil is treated with sulfuric acid, sodium hydroxide is used to neutralize the acid. This quote, for example, is lifted from an online patent application:

Turkey Red Oil is prepared by adding concentrated sulfuric acid (93%) to castor oil at 25°-30°C. From about 15 to 30% sulfuric acid is added to the castor oil. After all of the acid has been added and reaction is complete, the reaction product is washed with water and then neutralized with sodium hydroxide solution.

Isn't that interesting! My author acquaintance may be right on the money. Note that he said "a SAP value close to 0," not a SAP value of 0. For my purposes, though, close to 0 is close enough.